This invention concerns ignition plugs, such as spark plugs and plasma jet ignition plugs, for igniting fuel. Such plugs generally comprise two spaced apart electrodes, usually a center electrode and a ground electrode, across which an electric arc discharge is drawn for ignition. Examples of spark plugs are shown in U.S. Pat. Nos. 4,926,088, 4,808,135, 4,725,254, 4,684,352, 4,606,730, 4,585,421, 4,575,343, 4,374,450, 4,345,179 and 3,356,882. Examples of plasma jet ignition plugs are shown in U.S. Pat. Nos. 4,766,855, 4,760,820, 4,493,297, 4,487,192, 4,471,732, 4,396,855 and 4,337,408.
In the prior art, the arc occurs anywhere on the end face of the center rod electrode, including the peripheral edge thereof. When the arc occurs at the edge, the electrode is more subject to erosion than when the arc is kept away from the edge. It is a purpose of this invention to provide an electrode that keeps the arc away from the edge.